Field of the Invention
The invention pertains to methods of making composite electrodes for lithium ion batteries, and more particularly, to methods of fabricating composite cathodes suitable for both liquid cell and all-solid-state cell applications, and batteries containing the same.
Description of Related Art
Electrodes, especially the cathodes, for traditional lithium ion batteries are typically multi-component structures. They include: nanoparticles of the active cathode material for lithium storage; an electron conductor that is either carbon black, carbon nanotube, carbon fiber, or graphene; a binding agent that is an insulating polymer that binds all the nanoparticles to each other and to a substrate; and an ionic conductor that is usually provided by forming the film of the composite of other components deposited on a metallic current collector foil and then soaking in a liquid electrolyte.
The active material nanoparticles as well as the nanoparticles of conductive carbonaceous materials are performed. To improve the cell performance, researchers over the years have worked on size distribution of the nanoparticles, doping of the active material nanoparticles with other elements, and coating the active material nanoparticles with an electronic conductor film or an ionic conductor film. Several of these methods as previously disclosed include:
U.S. Pat. No. 7,608,362 describes a method of producing a composite cathode active material powder comprising at least one large diameter active material selected from the group consisting of metal composite oxides and at least one small diameter active material selected from the group consisting of carbon-based materials and metal oxide compounds. Mixing the large and small diameter active materials in a proper weight ratio improves packing density; and including highly stable materials and highly conductive materials in the composite cathode active materials improves volume density, discharge capacity and high rate discharge capacity. The large diameter active material is selected from the group consisting of compounds LixCo1−yMyO2−αXα and LixCo1−y−zNiyMzO2−αXα, and at least one small diameter active material is selected from the group consisting of compounds represented by LixCo1−y−zNiyMzO2−αXα, LixMn2−yMyO4−αXα, and LixCo2−yMyO4−αXα, Where M is selected from the group consisting of Al, Ni, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and mixtures thereof, and X is selected from the group consisting of O, F, S, P, and combinations thereof, and carbon-based material. The carbon-based material may be selected from the group consisting of graphite, hard carbon, carbon black, carbon fiber, carbon nanotubes (CNT) and mixtures thereof.
U.S. Pat. No. 7,842,420 describes a method of fabricating powder of cathode material from a starting mixture which includes a metal, a phosphate ion, and an additive which enhances the transport of lithium ions in the resultant material. The cathode material comprises LixMPO4 wherein M is metal such as iron, and x ranges from 0 upwards to approximately 1, and the additive is selected from the group consisting of: V, Nb, Mo, C, and combinations thereof. The additive may function as a nucleating agent which promotes the growth of at least one component of the material. In still other instances, the additive may promote the reduction of a carbon-containing species in the starting mixture so as to generate free carbon, and this free carbon may be at least partially sp2 bonded. In yet other instances, the additive is operative to modify the lattice structure of the material so that the transport of lithium ions through the modified lattice is enhanced in relation to the transport of lithium ions through a corresponding unmodified lattice. The mixture is heated in a reducing environment to produce the material then ball milled to produce the powder. Heating may be carried out in a temperature range of 300-750° C.
U.S. Pat. No. 7,396,614 describes a method of fabricating a composite positive electrode material comprising a non agglomerating lithium vanadium oxide particles, of the form Li1+xV3O8 in which 0.1≦x≦0.25, as active material, a carbon black material which confers electron conduction properties to the electrode, and a mixture of lithium salt and organic binder which confers ionic conduction properties and mechanical properties to the electrode. The composite positive electrode can be prepared by mixing the active material and the carbon black in a solution of the binder and lithium salt in an appropriate solvent and then by evaporating the solvent under hot conditions under a nitrogen atmosphere. The process for the preparation of the active compound consists in reacting at least one Li precursor with at least one vanadium precursor. The lithium precursor can be chosen from lithium oxides such as Li2CO3, LiNO3, LiOH, LiOH.H2O and Li2O and organic lithium salts, such as lithium acetylacetonate, lithium acetate, lithium stearate, lithium formate, lithium oxalate, lithium citrate, lithium lactate, lithium tartrate or lithium pyruvate. The vanadium precursor can be chosen from vanadium salts and vanadium oxides such as α-V2O5, NH4VO3, V2O4 and V2O3.
U.S. Pat. No. 7,923,154 describes a method of synthesis of carbon-coated powders having the olivine or NASICON structure. Carbon-coating of the powder particles is necessary to achieve good performances because of the rather poor electronic conductivity of NASICON structures. For the preparation of coated LiFePO4, sources of Li, Fe and phosphate are dissolved in an aqueous solution together with a polycarboxylic acid and a polyhydric alcohol. Upon water evaporation, polyesterification occurs while a mixed precipitate is formed containing Li, Fe and phosphate. The resin-encapsulated mixture is then heat treated at 700° C. in a reducing atmosphere to produce a fine powder consisting of an olivine LiFePO4 phase, coated with conductive carbon. This powder is used as active material in a lithium insertion-type electrode.
U.S. Pat. No. 7,892,676 describes a method of producing a cathode material comprising a composite compound having a formula of A3xM12y(PO4)3, and a conductive metal oxide having a formula of M2aOb, wherein A represents a metal element selected from the group consisting of Groups IA, IIA and IIIA; each of M1 and M2 independently represents a metal element selected from the group consisting of Groups IIA and IIIA, and transition elements. The cathode material is prepared by the following steps: preparing a solution including A ion, M1 ion, and PO43−; adding M2 salt into the solution; adjusting the pH of the solution so as to form M2 hydroxide and to convert M2 hydroxide into M2 oxide; and heating the solution containing M2 oxide so as to form the cathode material with fine particles of M2 oxide dispersed in an aggregation of particles of A3xM12y(PO4)3.
U.S. Pat. No. 7,939,198 describes a method to produce a composite cathode comprising an electroactive sulfur-containing cathode material that comprises a polysulfide moiety of the formula —Sm—, wherein m is an integer equal to or greater than 3; and an electroactive transition metal chalcogenide having the formula MjYk(OR)l wherein: M is a transition metal; Y is the same or different at each occurrence and is oxygen, sulfur, or selenium; R is an organic group and is the same or different at each occurrence; j is an integer ranging from 1 to 12; k is a number ranging from 0 to 72; and l is a number ranging from 0 to 72; with the proviso that k and l cannot both be 0. The chalcogenide encapsulates the electroactive sulfur-containing cathode material and retards the transport of anionic reduction products of the electroactive sulfur-containing cathode material. The method relates to the fabrication of a composite cathode by a sol-gel method wherein the electroactive sulfur-containing cathode material, and optionally binders and conductive fillers, are suspended or dispersed in a medium containing a sol (solution) of the desired electroactive transition metal chalcogenide composition; the resulting composition is first converted into a sol-gel (e.g., a gel-like material having a sol-gel structure or a continuous network-like structure) by the addition of a gelling agent, and the resulting sol-gel is further fabricated into a composite cathode.
All the approaches above still require an organic binder to bind the various nanoparticles together among themselves and to the substrate or current collector. The liquid electrolyte that permeates the cathode made up of lithium storage particles, electron conducting particles, the film of insulative organic binder surrounding the particles, and the voids provides lithium ion conduction. Thus the transport of lithium ion from the liquid and the energy storage particles is limited by the surrounding insulative binder film; this leads to local solid electrolyte interface (SEI) layer formation around the particles because of the side reaction taking place between the liquid electrolyte and organic binder film. The continuous adverse change in the properties of this SEI layers limit the performance and the lifetime of the traditional lithium ion cells.
J. S. Wang et al. [Journal of Power Sources 196:8714-18 (2011)], tried to increase the specific energy density of traditional cells. The cell has a cathode consisting of 1.2 mm thick Al foam filled with a slurry composed of 84 wt. % Li(NiCoMn)1/3O2 (L333, NCM-01ST-5, Toda Kogyo)+9 wt. % poly(vinylidene fluoride-cohexafluoropropylene) binder (Kynar Flex 2801, Elf Atochem)+3.5 wt. % carbon black (Super P, MMM)+3.5 wt. % synthetic graphite (KS6, Timcal); an anode, made using 1.2 mm thick Cu foam filled with the slurry of 93 wt % active carbon material (SG, Superior Graphite, SLC 1520), 3 wt. % carbon black (Super P), and 4 wt. % SBR binder (an aqueous styrene-butadiene rubber binder, LHB-108P). The best performance of 10 mAh/cm2 was obtained only at low C rate C/50. A rapid fade was observed at C rate as low as C/20. The energy density of the cell is low because of thick electrodes, also the fundamental low cycle life affecting the traditional cell due to SEI layer has not been addressed by this approach.
In recent years, attempts have been made in making binder free and liquid electrolyte free cathodes in cells as reported by the following:
Hayashi, et al. [Journal of Power Sources 183:422-26 (2008)] constructed a laboratory-scale solid-state cell consisting of the composite cathode powder obtained by mixing Li2S—Cu materials, the lithium ion conductor 80Li2S.20P2S5 glass-ceramic, and electronic conductor acetylene-black with the weight ratio of 38:57:5. The composite powder (10 mg) as a cathode, and the 80Li2S.20P2S5 glass-ceramic powder (80 mg) as a solid electrolyte were placed in a polycarbonate tube (with a diameter of 10 mm) and pressed together under 3700 kg/cm2, and then an Indium foil as a negative electrode was pressed under 1200 kg/cm2 on the pellet. After releasing the pressure, the obtained pellet was sandwiched by two stainless-steel rods as current collectors. The cells were charged and discharged at room temperature in an Ar atmosphere using a charge-discharge measuring device (BTS-2004, Nagano). The constant current density of 64 μA/cm2 was used for charging and discharging with the maximum discharge capacity of 490 mA-h/g.
Sakuda et al. [Chem. Mater., Vol. 22, No. 3, 2010] constructed all-solid-state cells as follows. Mixing Li2SiO3 coated LiCoO2 and the 80Li2S3-20P2S5 glass-ceramic electrolyte with a weight ratio of 70:30 using an agate mortar to prepare composite positive electrodes. A bilayer pellet consisting of the composite positive electrode (10 mg) and glass-ceramic solid electrolytes (80 mg) was obtained by pressing under 360 MPa in a 10 mm diameter tube; indium foil was then attached to the bilayer pellet by pressing under 240 MPa. The pellet was pressed using two stainless steel rods; the stainless steel rods were used as current collectors for both positive and negative electrodes. All the processes for preparation of solid electrolytes and fabrication of all-solid-state batteries were performed in a dry Ar-filled glovebox ([H2O]<1 ppm). A discharge capacity of 60 mAh/g was obtained at a discharge current density of 64 μA/cm2 at 30° C.
Also, Sakuda et al. [Journal of Power Sources 196:6735-41 (2011)]; using the same cell construct described above, used LiCoO2 composite cathode, where LiCoO2 was coated with LiNbO3 then 80Li2S3-20P2S5 films; these particles where then mixed with 80Li2S3-20P2S5 particles to form the composite cathode. The resulting best cell was charged/discharged at the current density of 0.13 mA/cm2 and gave a discharge capacity of 95 mA-h/g.
Importantly, the LiCoO2 particle coating was done with Pulse Laser Deposition (PLD), a process that is relatively unsuitable for routine manufacturing. And all the solid state cells were made by pressing the stack of powder of various components into small area cylindrical disk, a cell fabrication technique that is not readily scalable. The mechanical contact between the particles that dependent on pressing pressure provides less than ideal electrical contact between various particles. The latter combined with too thick solid state electrolyte layer in the cell leads to undesirable overall cell impedance that limits the extractable capacity.
What is needed, therefore, is a scalable, efficient process for making composite cathodes for lithium ion batteries that is suitable for use in both liquid cell and all-solid-state cell applications.
Objects and Advantages
Objects of the present invention include the following: providing an improved composite electrode for lithium ion batteries; providing a composite cathode for alkali ion batteries; providing a composite cathode suitable for both liquid cell and all solid state metal ion batteries; providing an improved alkali ion battery; providing methods for fabricating composite electrodes for metal ion batteries; and providing a scalable, manufacturable process for making composite electrodes and batteries containing them. These and other objects and advantages of the invention will become apparent from consideration of the following specification, read in conjunction with the drawings.